A power converter employing an electric power switching element has steadily broadened a range of applications owing to increased capacity and speed of the switching element. Among electric power switching elements, especially, an insulating gate bipolar transistor (IGBT) and a metal-oxide semiconductor field effect transistor (MOSFET), both of which are an MOS gate type switching element, have stretched out to a various field of applications.
The IGBT and MOSFET are a non-latching type switching element in which an on/off state is not self-sustained, so that they have a great advantage over a latching type switching element such as a thyristor or the like in that higher controllability due to a gate drive is ensured. Such a non-latching type switching element is unrestrainedly able to prevent an occurrence of a surge voltage or current and regulate a current or voltage gradient during a switching transient period owing to gate-control thereof.
By the way, as a gate driving circuit configured to protect an electric power switching element from a surge current by gate-control, there has been known one comprised of an electric power switching element including a sensing terminal that shunts part of a main current from a gate terminal to be used to input a control signal as a current-detection current, a computing means that deducts a certain amount of current from the current-detection current shunted by the sensing terminal, an accumulating means that accumulates the remaining current after the certain amount of current is deducted by the computing means when a value of the remaining current is positive, and a controlling means that controls a gate voltage of the electric power switching element based on the output from the accumulating means (See Japanese Patent Application Laid-open Publication No. 2000-224837, for example). According to this gate driving circuit, the main current flowing through the electric power switching element is stably controlled without relying on feedback control.
The above gate driving circuit protects from a surge current the electric power switching element having the sensing terminal as well as a gate terminal by controlling the gate terminal without relying on feedback control. However, an electric power switching element having not the sensing terminal but only a gate terminal is generally protected from a surge voltage or the like by employing feedback control to control the gate terminal.
As an example of a conventional gate driving circuit that controls a gate terminal to protect an electric power switching element from a surge voltage or the like by employing feedback control, there has been known one shown in FIG. 1, for example. By supplying a gate signal output to a gate electrode (control input terminal) of an electric power switching element 9 from a voltage amplifier 2 via a gate resistor 3, a gate drive of the electric power switching element 9 is carried out under a normal operation condition. At the same time, by supplying a current to the gate electrode of the electric power switching element 9 from a control current-source 6 as a current drive means, an active gate drive of the electric power switching element 9 is carried out. In this case, the control current-source 6 is controlled by a voltage amplifier 5 in the following way. That is, the voltage amplifier 5 inputs a divided voltage detection signal obtained by dividing a principal voltage Vce applied across principal electrodes (collector-emitter) of the electric power switching element 9 by use of voltage-dividing resistors 4a, 4b and thus controls the control current-source 6. A reference mark 10 designates a flywheel diodes connected parallel with the electric power switching element 9.
In the above gate driving circuit, a surge voltage generated when the electric power switching element 9 turns off is suppressed in the following manner. An external signal coming thereinto affects the voltage amplifier 2 so as to reduce the output voltage thereof, which then reduces the gate voltage applied thereto via the gate resistor 3. When the gate voltage of the electric power switching element 9 becomes lower than a threshold voltage of a certain value determined in accordance with a static characteristic of the switching element, the current flowing through the electric power switching element 9 starts to reduce and at the same time the principal voltage Vce of the electric power switching element 9 starts to increase abruptly.
The principal voltage Vce is divided by the resistors 4a, 4b and phase compensation capacitors 13a, 13b. The divided voltage as a principal voltage detection signal is input into the voltage amplifier 5 and amplified to an appropriate level therein, thereby controlling the control current-source 6. Therefore, when the principal voltage Vce exceeds a predetermined value, the current injected from the control current-source 6 makes the gate voltage of the electric power switching element 9 increase. Then, the electric power switching element 9 returns back to an on state, thereby preventing a further increase in the principal voltage Vce. Such operation by the gate driving circuit above prevents the principal voltage applied to the electric power switching element 9 from exceeding a certain value determined by the main circuit, thereby suppressing an occurrence of a surge voltage.
The gate driving circuit employing an electric power switching element having the sense terminal as well as the gate terminal protects the electric power switching element from a surge current by controlling the gate terminal of the electric power switching element without feedback control. However, an electric power switching element having not a sensing terminal but only a gate terminal is generally protected from a surge current or the like by controlling the gate terminal through feedback control.
In such a gate driving circuit by controlling the gate terminal through feedback control to protect the electric power switching element from a surge voltage or the like, one end of the control current-source 6 is connected to a positive terminal of a gate drive power source 1c as illustrated in FIG. 1. Such connection is based on the following reason.
The gate-emitter voltage Vge of the electric power switching element 9 is determined by gate drive power sources 1a, 1b. The voltage value is about +/−15 volts in a majority of switching elements. The control current-source 6 is required to feed a certain amount of current determined through a control of the voltage amplifier 5 into the gate electrode of the electric power switching element 9, regardless of the gate-emitter voltage Vge.
Unless this current is kept constant, the output current from the control current-source 6 is fluctuated by fluctuation of the gate-emitter voltage Vge, which may become an error factor in feedback-controlling the principal voltage Vce. Therefore, the power source voltage of the control current-source 6 has to be as high as possible compared to the gate-emitter voltage Vge. In fact, there is required a power source that can afford a voltage of from several to less than 20 volts as the gate drive power source 1c, taking account of a loss in the circuit. As explained, the conventional art as illustrated in FIG. 1 requires a power source that affords a higher voltage, let alone the gate drive power source per se, for a gate control during a transient period.
In addition, in the gate driving circuit illustrated in FIG. 1, the principal voltage Vce is divided by the voltage-dividing resistors 4a, 4b and the phase compensation capacitors 13a, 13b in order to obtain a principal voltage detection signal in proportion to the principal voltage Vce and the divided voltage as an input signal is amplified by the voltage amplifier 5. In such configuration, the voltage amplifier 5 has to have a higher input impedance in order to reduce an electric power loss. That is, if the input impedance is lower, a resistance of the voltage-dividing resistors 4a, 4b needs to be lower; and thus the resistors 4a, 4b having a lower resistance cause a larger power loss therein in case of a voltage converter in which the principal voltage Vce to be applied to the electric power switching element 9 reaches to several thousands volts.
The present invention has been made in view of the above and the objective thereof is to realize a power source for use in an active gate drive means having a simple configuration at a low cost. Another objective of the present invention is to provide a gate driving circuit that enables a lower cost operation of a control signal detection means configured to carry out an active gate control.